Joshua 4:12

And the children of Reuben, and the children of Gad, and half the tribe of Manasseh, passed over armed before the children of Israel, as Moses spake unto them:

The children of Reuben ... passed over armed before the children of Israel. There is no precedency to the other tribes indicated herethere is no reason to suppose that the usual order of march was departed from; but these are honourably mentioned, to show that, in pursuance of their engagement (Joshua 1:16), they had sent a complement of fighting men to accompany their brethren in the war of invasion.